Thermoplastic resins, in particular polycarbonate-based resins, are widely used in or as electric/electronic parts, OA equipment, domestic articles or building materials owing to their good impact resistance, heat resistance, and electric characteristics, among others. Whereas polycarbonate-based resins have higher flame retardancy as compared with polystyrene resins and the like, there are fields in which a still higher level of flame retardancy is required, mainly in the fields of electric/electronic parts, OA equipment, and the like. Thus, improvements in such flame retardancy have been attempted by addition of various flame retardants. For example, the addition of organohalogen compounds and organophosphorus compounds is conventional in the art. However, most of organohalogen compounds and organophosphorus compounds have a problem concerning toxicity and, in particular, organohalogen compounds have a problem in that, upon combustion, they generate corrosive gases. Under such circumstances, the demand for halogen-free and phosphorus-free flame retardants for rendering such resins flame-retardant has been increasing in recent years.
The utilization of polyorganosiloxane compounds (also called silicones) as halogen-free, phosphorus-free flame retardants has been proposed. For example, Japanese Kokai Publication Sho-54-36365 describes that flame-retardant resins can be obtained by kneading together a silicone resin comprising monoorganopolysiloxane and a nonsilicone polymer.
Japanese Kokoku Publication Hei-03-48947 describes that mixtures of a silicone resin and a group IIA metal salt confer flame retardancy upon thermoplastic resins.
Japanese Kokai Publication Hei-08-113712 describes a method of obtaining flame-retardant resin compositions by dispersing, in a thermoplastic resin, a silicone resin prepared by mixing up 100 parts by weight of a polyorganosiloxane and 10 to 150 parts by weight of a silica filler.
Japanese Kokai Publication Hei-10-139964 describes that flame-retardant resin compositions can be obtained by adding a solvent-soluble silicone resin having a weight average molecular weight of not less than 10,000 but not more than 270,000 to a nonsilicone resin containing aromatic ring.
However, the silicone resins described in the above-cited documents are effective in conferring flame retardancy only to an unsatisfactory extent. An increase in the amount of flame retardant for attaining satisfactory results will cause deterioration in impact resistance of the resulting resin compositions and, thus, there is still a problem such that it is difficult to obtain resin compositions with both excellent flame retardancy and good impact resistance.
Japanese Kokai Publication 2000-17029 describes that flame-retardant resin compositions can be obtained by incorporating in a thermoplastic resin a composite rubber type flame retardant resulting from graft-polymerizing a vinyl monomer onto a composite rubber composed of a polyorganosiloxane rubber and a poly(alkyl (meth)acrylate) rubber.
Japanese Kokai Publication 2000-226420 describes that flame-retardant resin compositions can be obtained by incorporating in thermoplastic resins a polyorganosiloxane type flame retardant resulting from grafting a vinyl monomer onto composite particles comprising a polyorganosiloxane having aromatic group and a vinyl polymer.
Japanese Kokai Publication 2000-264935 describes that flame-retardant resin compositions can be obtained by incorporating in thermoplastic reins a polyorganosiloxane-containing graft copolymer resulting from graft polymerizing a vinyl monomer onto polyorganosiloxane particles not larger than 0.2 μm in size.
The flame-retardant resin compositions described in Japanese Kokai Publication 2000-17029, Japanese Kokai Publication 2000-226420 and Japanese Kokai Publication 2000-264935 all have satisfactory levels of impact resistance but are not satisfactory in flame retardancy. Thus, they still have a problem in that they are not excellent both in flame retardancy and in impact resistance.
Further, while Japanese Kokai Publication 2000-264935 described that the polyorganosiloxane-containing graft copolymer can be recovered in the form of a powder by spray drying, there is no specific example of the composition is given. As far as the investigation made by the present inventors indicates, such a polyorganosiloxane-containing graft copolymer as recovered by spray drying is good in flame retardancy but has another problem, namely it is poor in powder characteristics as compared with the grade of copolymer recovered by salt coagulation, in particular it is unsatisfactory in anti-blocking property.